Cameras utilized in providing an image of a radioactive source are frequently composed of a scintillator crystal and a collimator for guiding radiant energy from the source to the scintillator crystal. An array of photomultipliers is positioned on the opposite side of the scintillator crystal for receiving a flash of light emitted by the crystal in response to the incident radiant energy. Typically, the measure of light energy received by each photomultiplier is obtained by integrating circuits coupled to each of the photomultipliers, the relative magnitudes of these energies serving to indicate the location of each of the light flashes on the crystal. The stored energy of each of the photomultipliers is converted to a signal suitable for combining with the signals obtained from the energies of the other photomultipliers. In particular, it is noted that a highly active source of high energy radiation rapidly illuminates the scintillator crystal with successive photons of high radiant energy such as the energy of x-rays or gamma rays. Such rapid illumination is useful in obtaining high resolution images of the source, provided that the electronic circuitry utilized in forming pulse signals from the energy obtained from the photomultipliers is capable of operating at a rate commensurate with the rate at which high energy photons are incident upon the scintillator crystal.
A problem arises because of the need for integrating the light energy from the flash of light obtained in response to each high energy photon, and because of the need for forming a suitable signal from the integrated energy to permit a combination of these signals for locating the positions of the flashes of light. More particularly, the light output from a typical NaI with thallium doping scintillator may be characterized by a period of approximately constant light output for approximately 150 nanoseconds (ns) followed by a fast decay mode with a time constant of 230 ns. There is, in addition a slow decay mode with a time constant of 1200 ns. The light pulse is typically converted into a current pulse by the photomultipliers. Approximately 90 percent of the light energy is collected by the photomultiplier tube in approximately 800 ns and, in response to such collected light energy a current pulse is produced. An integrator is fed by the current pulse to produce a pulse having an amplitude proportional to the energy in the current pulse. The integrator is typically a so called "leaky" integrator having a decay time constant adjusted so that the output pulse it produces peaks at 800 ns. after the start of the current pulse i.e. when about 90 percent of the light energy is collected by the photomultiplier. Since the high energy photons are sometimes received less than 1.5 to 2.0 nanoseconds apart it is necessary that the integration provided by the integrator be complete prior to the arrival of the next high energy photon to enable the camera to provide high image resolution and high count rate capability.